Certain Dark Things
by Emjen Enla
Summary: [Complete] Post-CoFA. He looked down at himself. The rune Lilith had put on his chest was still there, bright red, but he felt no panic. He was in control of his own body now. The spell was broken. Everything was okay. Or Jace and Sebastian in the aftermath of Sebastian's resurrection.


**I don't own the Shadowhunter Chronicles. Title from "Sonnet XVII" by Pablo Neruda, which is also the title of part two of CoLS. Violence warning because these boys spend a not inconsiderable amount of time literally covered in Jace's blood.**

 **I'm honestly not sure why I wrote this since CoLS is my least favorite book in the series mostly because I don't like not-Jace. I guess I'm rationalizing this story by saying that writing not-Jace will be good practice. Plus, Sebastian is a great villain. I'm not sure if I did him justice, but I tried.**

 **For the purpose of this story, just accept that Jace has a stele through the entirety of the climax of CoFA and that Sebastian is wearing Valentine's teleporting ring. Trying to figure out how two blood-soaked teenage boys could get into the apartment and not get caught by the Clave without those two things gives me a headache.**

* * *

Jace Lightwood woke up.

That was admittedly a little strange because he hadn't been asleep, but somehow, he woke up anyway. He was kneeling in a glass box up to his waist in water that had turned red with blood. His blood. He remembered cutting open his own wrist and letting the blood run into Sebastian Morgenstern's mouth. He remembered doing it, remembered not being in control of his own body.

He looked down at himself. The rune Lilith had put on his chest was still there, bright red, but he felt no panic. He was in control of his own body now. The spell was broken. Everything was okay.

He still felt strange, almost floaty, like the inside of his head had been rearranged and pushed around, like his thoughts weren't quite his own. When he tried to consider the feeling more closely, a sense of peace swept over him. It was alright. Everything was alright now. It was a just a weird feeling, probably just because of all the blood he'd lost.

The surface of the water bucked, and Sebastian sat up, eyes wide, bloody water pouring off his body, sticking his colorless hair to his face and neck. Jace jerked back, more because he was startled than because he was actually afraid. Sebastian had just been dead and now he wasn't; that was a little weird. Jace was allowed to feel startled even if Sebastian wasn't a threat.

Sebastian gripped the sides of the box with desperate strength. At some point his missing hand had grown back, or perhaps been reattached was a better word because there was a ropey scar where Isabelle had sliced it off. Sebastian leaned forward, eyes squeezed tight closed. He coughed and retched up the combination of water and Jace's blood that he'd been submerged in. His sides expanded and contracted in a ragged, irregular pattern, like he'd forgotten how to breathe. There was a slash on one of his palms right where Jace had cut himself mere minutes before.

Jace leaned forward, stretching out his hand carefully, not sure if Sebastian would be okay with being touched. "Are you okay?" He asked, and Sebastian motioned vaguely with one hand. The motion shouldn't have meant anything, but Jace somehow knew Sebastian was saying he was fine.

Jace realized he was really worried about Sebastian. That was startling. Jace had been trying to keep Lilith from bringing Sebastian back, and now all he could think about was how Lilith had been planning to use Simon's blood to finish the resurrection. What if Jace's blood wasn't good enough? What if there was something wrong with Sebastian as a result?

It was the same type of thought-spiral that happened every time Alec was injured. Jace was a bit confused by why he suddenly cared about Sebastian as much as he did his _parabatai_. Wasn't Sebastian the enemy?

No, Sebastian wasn't. He realized. It was all a big misunderstanding. Sebastian hadn't meant to do those things. Max's death had been an accident. Sebastian hadn't meant to kill him. He was trying to do the right thing. What he wanted was right.

Jace couldn't remember when exactly he'd learned all this information, but he knew it was true. He was more certain of it than he was of just about anything else. Sebastian was good, and Jace needed to help him.

"Are you sure you're alright?" Jace asked again. "Do you need an _iratze_?"

"No, I'm fine," Sebastian said, his voice gravelly like he'd swallowed a throatful of stones. "You're the one who needs an _iratze_."

"What?" Jace blinked in confusion. "What are you talking about?"

"Your arm," Sebastian pointed with one finger, seemingly unwilling to fully let go of the box.

Jace looked down. Under Lilith's compulsion, he'd cut into his arm very deeply. The wound was still bleeding, but not anywhere near as heavily as the severity of the wound would suggest. Perhaps that had something to do with the spell that had just been completed.

"Fix yourself up before you bleed to death or something," Sebastian said. "Then we'll get out of here."

"Alright," Normally Jace would have argued that he was fine, but strangely he had no desire to argue with Sebastian. Sebastian knew what was best.

He got to his feet and his head spun. He grabbed for the side of the box and someone grabbed his hand and held him up. When Jace's vision cleared he found himself staring down at Sebastian. The other boy's black eyes were not necessarily concerned, but he was still there helping to hold him up.

"Patch yourself up," Sebastian said, letting go once he was sure Jace wasn't going to collapse.

Jace clambered out of the box, found his _stele_ and scrawled an _iratze_ and a blood replacement rune onto his wrist. When he turned back, Sebastian was climbing gingerly out of the box, looking like he didn't quite trust his legs.

"Jonathan," Jace said, opting for that name because he wasn't sure how the other boy would feel about being called Sebastian. "What happens now?"

Sebastian glared at him. "Don't call me that. It's not my name any more than its yours."

"Then what am I supposed to call you?" Jace asked. "Sebastian?"

"That'll do," Sebastian said. "Give me your _stele_ and we'll get out of here."

"What about Clary?" Jace asked.

"Clary," Sebastian repeated. There was an odd expression on his face, not necessarily unfamiliar, just not the kind of expression someone normally got when thinking about their sister. Jace tried to place the expression for a minute then the curiosity floated away. It didn't matter what Sebastian thought about Clary. He would never hurt anyone Jace cared about, not on purpose.

"Can she come with us?" Jace asked. "I want her to come with us."

"How can you be sure we can trust her?" Sebastian raised a cynical eyebrow at Jace.

"She loves me. If I ask her to come help us, she will," Jace wasn't sure when his relationship with Clary had become so simple. He couldn't remember ever being this certain about anything even remotely concerning her, but he wasn't necessarily unset by the change. It was nice to think about Clary without drowning in a tidal wave of guilt and confusion.

"I'm glad you trust her so completely, Brother," Sebastian said, "but we need to be careful. We will escape and heal and keep an eye on Clary. If she proves to be trustworthy we can get her then."

"Alright," Jace said, vaguely aware that he was agreeing too easily, but not particularly caring. He paused for a minute considering something else Sebastian had said. "Did you just call me 'Brother?'"

"Yes," Sebastian said. "We were both raised by Valentine Morgenstern, in a way that makes us brothers, does it not?" he paused for a moment then asked, "Does it bother you that I called you that?"

"No," Jace said. It would have bothered him only a few minutes before, but now he was deeply thankful for Sebastian's implicit trust and intimacy. He'd literally killed this boy and tried to keep Lilith from giving him another chance at life, he couldn't believe Sebastian had forgiven him. "You can call me that if you want, I would be honored."

Sebastian looked thoughtfully at him for a minute. He looked mildly confused, just as Jace felt, but after a moment the expression was gone. "Give me your _stele_ ," he said. "The Clave is downstairs, and it will only be a few minutes before they make their way up here. We don't want to be here when that happens."

Jace handed over the _stele_. He was curious about how Sebastian planned to get away without running into the Clave, but he didn't question him. He knew Sebastian would take care of it, Jace just needed to trust him.

Sebastian strode across the garden motioning for Jace to follow. Sebastian walked with purpose, but he was obviously unsteady. Jace ran a couple sets to catch up and gripped the other boy's arm to give him some support. Sebastian gave him a look that was part confusion, part threat then relaxed. "Sorry," he said. "This is… strange."

Jace nodded in agreement. He knew that what was happening right now was right, but he still wasn't sure how it he had reached this point, and that was odd and a little unsettling. "You said we needed to go," he said quietly.

Sebastian nodded curtly and continued on. Jace stayed with him until they reached a wall. Then Sebastian moved suddenly using his arm to pin Jace's hand against his side. Years of training made Jace stiffen, but he wasn't really alarmed. "Sebastian?" he asked.

Sebastian didn't respond, he reached to touch a ring on one of his fingers. There was a flash of silver light and Jace felt like he was being pulled after Sebastian into something, condensed down into nothing and shot through the air. A split second later Jace's vision cleared and he and Sebastian were standing a horribly familiar cave system. It was near instantaneous travel, but it was nothing like portaling.

"Sebastian," Jace asked again. "Where are we?"

Sebastian grinned, but the expression was a tried one, like it took energy to perform it. "I think you already know that."

Jace knew. This was the cave he had followed Sebastian to several weeks before, the one where Valentine had released the demons from. "Why are we here?" he asked.

"Because this is where the door is," Sebastian said cryptically, then he lifted Jace's _stele_ and drew a rune on the cave wall. After a second he pulled Jace forward and stepped _into_ the wall. Jace stumbled after and found himself in an ultramodern apartment. He had no idea where they were, but somehow, he knew it was nowhere near the rooftop they had just been on.

"What?" Jace looked around, taking in the sleek black and white decor. There were no windows. "Where are we now?"

"This was our father's safehouse," Sebastian let go of Jace and pressed the _stele_ back into his slack hand. "This is where we lived for years after he sent you to live with the Lightwoods." There was no jealousy or gloating in his voice when he talked about the differences in the way Valentine had treated them. Jace didn't think that was odd. He and Sebastian hadn't understood each other before, they did now there was no need for jealousy now.

"Will they find us here?" Jace asked.

"Technically there's no 'here' for them to find," Sebastian crossing the room to lean heavily on the huge glass table. "This is an interdimensional pocket; the only way out is that stretch of wall behind you. They can't track us, and they definitely can't find the entrance let alone figure out how to open it. There is no safer place to be."

Vaguely, Jace was aware that he probably should have been nervous since he was basically trapped in a place that wasn't even part of his world, but he wasn't. He trusted Sebastian. Everything would be okay.

"We should rest," Sebastian ran a hand down his face, smearing Jace's half-dried blood down his face in even more ghastly patterns. "We're both exhausted."

"I'm fine," Jace said, automatically.

Sebastian gave him a look. It wasn't like Alec's exasperated but concerned expressions, it was the look of someone who knew he was being lied to and didn't like it. There wasn't any overt concern, but Jace still found it comforting. "You look terrible," Sebastian said, matter of fact. "You look like you haven't slept in the entire time I've been dead. My sister must be a fool if she didn't notice."

"She had other things on her mind," Jace said. _Things like worrying I was going to break up with her._ Strangely, the thought wasn't as painful as it would have been just hours before. Clary knew he loved her, she couldn't really have worried all that seriously about him leaving her.

"Regardless," Sebastian said. "You'll be a lot more useful if you don't look like you're about to collapse. There's a spare bedroom upstairs you can use. I've never been able to figure out if our father intended it for you or Clary, but I'm sure he's in a place where it doesn't matter either way."

Jace eyed the stairs heading upwards. "Are you sure you can make it up those stairs?"

Sebastian frowned then straightened up and let go of the table. He swayed visibly but didn't fall. "I'll make it," he said and headed for the stairs.

He did make it though Jace followed a step below just in case. At the top of the stairs, Sebastian gestured tiredly to one of the closed doors. "There's the room. There's an attached bathroom. I'll see you in the morning."

"Wait," Jace said as the other boy started to walk away.

"What?" Sebastian sounded tired and Jace felt bad for having to bother him.

"Do you have some clothes for me to borrow?" Jace gestured at his blood-soaked clothes. "These are ruined."

Sebastian stared blankly at him, then blinked as if he'd just realized what Jace was asking. "Oh, sure," he said and headed for another door, which presumably lead to his room.

The room was catastrophically messy. The clutter put Jace's teeth on edge, but the feeling was tinged with fondness like he felt when looking at Izzy's disaster zone of room.

Sebastian crossed the room and stumbled, catching himself on the dresser. He waved Jace away and pulled open one of the drawers. After a moment he yanked out ball of black clothes and tossed them at Jace. "There you go."

Jace caught the clothes and studied the other boy. "Are you going to be alright?" he asked.

"Yes, I'm fine!" Sebastian snapped. "I just came back to life I think I'm considerably better off than I was two hours ago. Go shower or something!"

Jace wanted to stay and make sure Sebastian didn't slip in the bathroom and crack his head open, but it was pretty obvious Sebastian wanted to be left alone to lick his wounds. Jace understood the feeling so he back out of the room and closed the door quietly behind him.

The room Sebastian had pointed out was simply furnished in comparison to the rest of the apartment with white painted walls and white sheets and blankets on the bed. It was calming after the messiness of Sebastian's room. Jace crossed to the attached bathroom and poked around in the drawers. The room seemed to have been prepared for someone to live there-Clary maybe? -because there was a toothbrush, toothpaste and a hairbrush in one of the drawers. There was also deodorant which was a relief. Using Sebastian's deodorant would have been far from the grossest thing Jace had ever done, but he was still glad not to actually have to do it.

There were a couple fluffy white towels hanging next to the shower and some shampoo and bodywash. Jace stripped out of his ruined clothes and stepped into the shower letting the hot water pound down onto his body.

After a lifetime of being a Shadowhunter Jace was used to washing blood, ichor and other disgusting things off himself. Like he always did, he gave himself a minute to soak, watching his own blood turn the water red. Once the minute was passed he lathered shampoo into his hair and began to scrub himself clean.

The shower water was warm and relaxing. Jace tried to wash himself as quickly and efficiently as possible but he was hit by the adrenaline crash before he finished. He leaned against the shower wall as the headache he'd been nursing for weeks slammed back into him. He ached, his stomach was vaguely upset, and he just felt nasty. The sad thing was that aside from some aches and pains he didn't feel that much worse than he had for the last few weeks. He was just so tired.

He finished showering with clumsy fingers and stumbled out of the shower. He dried off and reached for the clothes Sebastian had given him. There was a silky black pair of pajama pants and a black tee-shirt. The fact that they were twisted together did not bode well for their cleanliness, but Jace was too tired to care. The clothes didn't really fit, but thankfully Sebastian was both taller and broader across the shoulders than Jace was, so the clothes were too big not too small. Jace tied the drawstring of the pants tightly to keep them up and tried not to wonder how much weight he'd lost in recent weeks.

He knew that he should brush his teeth and hair, but he was too exhausted for that. He stumbled out of the bathroom and collapsed face down onto the bed. He lay there for a few minutes before he finally worked his way under the blankets. The bed was very soft and Jace was very tired; he was asleep before he even realized it was happening.

For the first time in weeks, he did not dream.

* * *

A hand poked Jace in the side, dragging him towards the surface of a deep ocean of sleep.

"Are you alive?" a voice asked.

Jace groaned and batted at the hand, not completely awake. "Go away," he grunted. He did not want to wake up. He hadn't slept well in weeks and he was so tired.

"Alright, fine," the person who was bothering him said. "You can just keep lying there comatose if that's what you want."

That was what Jace wanted. As soon as the annoyance was gone he sunk back into the ocean.

* * *

Eventually, Jace resurfaced in the waking world. Since the bedroom had no windows it was dark and there was no way to tell what time it was. After a moment, Jace reached out, turned the bedside lamp on and squinted at the clock sitting next to it. It was late morning, almost late enough to be considered lunchtime. He sunk back onto the pillows and stared up at the ceiling trying to gather himself. He hadn't slept that well in too long.

It took him almost twenty minutes to finally feel awake enough to get up. He felt groggy and heavy like his body had forgotten how to deal with actually getting restful sleep. He sat up very, very slowly and ran a tired hand down his face feeling stubble rasping against his palm. He hoped Sebastian had a razor he could borrow.

Eventually he pushed aside the blankets and stood. He padded out of the bedroom on tired feet. He felt a little hungover which was weird, but he was so thankful to have actually been able to sleep that he didn't care.

When he opened the bedroom door he heard all kinds of loud noises and music coming from the TV downstairs. Jace headed down the steps to see that Sebastian was sprawled on the couch watching what Jace thought was supposed to be an extremely violent movie.

"You know that's not what actually happens if you stab someone there," Jace said.

Sebastian craned his neck to look at Jace. "Oh, look he lives. Did you get enough sleep, Brother?" He voice was a little teasing, but in a familial way. "You've been dead to the world for over twenty-four hours."

For the first time Jace realized it was a new day. He blinked. "Oh…" he said. "I didn't realize…"

"Yes," Sebastian said, still with a friendly air of teasing. "That's obvious."

Jace rounded the coach, sat down and curled up against the arm of the couch. Sebastian raised an eyebrow at him, but Jace was the one who spoke first, "How are you feeling?"

"I've always been fine," Sebastian said rolling his eyes. "You're the one who's sporting the most obvious sleep hangover I've ever seen."

"Is that a real thing?" Jace asked.

"Google it," Sebastian said with a shrug then he got up. He was perfectly steady on his feet which made Jace a lot less worried about him. "The majority of the food in this place went bad while both Father and I were busy being dead, so now that you're awake I'm going to go out and get us some lunch."

"I can go with you," Jace said trying to straighten up.

"You're only nominally awake," Sebastian said. "And only the clothes you have are my pajamas, which do not fit you in case you hadn't noticed."

"Believe me, I've noticed," Jace said.

"Just stay here," Sebastian said. "I'll be back."

After he left, Jace lay on the couch and vacantly watched Sebastian's movie. It really wasn't the kind of thing he would have chosen to watch on his own, but it was what Sebastian had chosen and Jace wanted to get to know the other boy a little better. The confusion he'd felt the night before was gone and he was completely comfortable with his alliance with Sebastian. Everything was okay, this was right.

Twenty minutes later Sebastian came back with a plastic bag of paper-wrapped packages. "Roman pizza," he said grandly dropping the bag onto the coffee table in front of Jace. "Fresh from the source."

"Rome?" Jace said. "But this apartment is in Idris."

"You obviously haven't been paying attention," Sebastian threw himself back onto the couch. "This apartment was never _in_ Idris, just the door was. I've since moved the entrance to Rome, Italy. Have you ever been there?"

"No," Jace admitted. Some Shadowhunters travelled extensively, helping out wherever they went, but Jace had never had that opportunity. As a child he and Valentine had never left the Wayland Manor, something that made perfect sense now that he realized Valentine wasn't actually Michael Wayland. The Lightwoods had the Institute to contend with and didn't travel either. Jace had always secretly hoped that he, Alec and Izzy would be able to leave New York and see the world once they came of age, though it was seeming more and more like that would never happen. He'd been trying not to be disappointed, and now that he found he actually wasn't. It would be good to see the world with Sebastian, just like it would be good to see the world with Alec and Izzy.

"Here," Sebastian rummaged through the bag and tossed one paper wrapped package to Jace. Jace unwrapped it to reveal the type of Roman pizza which was baked in long strips and then cut and folded in half when the customer bought it. His stomach growled and he nearly sobbed in relief. The combination of stress, exhaustion and terror he'd been living with recently always stole his appetite, it had been awhile since he'd been hungry.

When he looked up Sebastian was watching at him with a raised eyebrow. "What?" Jace asked.

"You look like you're going to cry," Sebastian said, mostly teasing but also almost mocking.

"I'm fine," Jace said and stuffed a bite of pizza into his mouth before he did anything else embarrassing.

Jace inhaled several packages of pizza with barely a thought. When he finally tuned back into reality Sebastian was eating absently, his attention mostly focused on the movie. He glanced at Jace and his lips twisted into a smile. "Had enough to eat?"

Jace felt himself turn red. He normally didn't get embarrassed about eating, but he was well aware that he'd probably looked like he was eating for the first time in too long. Valentine had always been finicky about table manners, he had no way of knowing if Sebastian was too. "Yes," he said quietly. "Thank you."

Sebastian's smile was a little like the kind people gave to a puppy who had just done something cute and endearing. "Well, if my dear sister isn't going to take care of you properly, then I guess the job falls to me."

Jace found himself nodding along. On some level, he knew that there was something wrong with his unthinking agreement with Sebastian's casual belittling of Clary, but he didn't really think about it. It was really frustrating that Clary could spend so much time angsting about whether or not Jace still loved her but that Simon who didn't even like Jace had been the one to realize something was wrong and try to help. He hadn't thought about it at the time, but it was really annoying now that he wasn't panicking about what he might do if he spent too much time around her.

"I'm really thankful for all of this," Jace said feeling like the words needed to be spoken. "You have no reason to forgive me for everything I did to you while our father was still alive, but you did, and I'm can't express how thankful I am for that."

A confused expression crossed Sebastian's face, like he was thinking something he knew didn't make sense and couldn't figure out why he was thinking it, then his expression cleared. "You can thank me by helping me."

"What do you need help with?" Jace asked. He felt a little like he already knew, but he wanted to hear Sebastian say it, rather than make assumptions of his own.

Sebastian leaned back and stared at the television. He was debating how best to explain his goals, Jace somehow understood this almost instinctively.

"Tell me," Sebastian said after a while. "Have you ever heard the quote ' _Flectere si nequeo superos, Acheronta movebo_?'"

"Virgil," Jace said. "'If I cannot move Heaven, I will raise Hell.'"

Sebastian grinned, it was a sharp, wicked thing. "That, Brother, is exactly what we're going to do."

* * *

 **For some reason I love the "If I cannot move Heaven, I will raise Hell" quote so you know I had to get it in here.**

 **You know it occurs to me that this is the first Jace story I've ever written that isn't angst...**

 **Please fav, follow and comment!**

 **Emjen**


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